Submerged arc welding (also called SAW or sub arc) is a type of arc welding where the arc is not visible. Sub arc welding produces coalescence of metals by heating them with an arc between a bare metal electrode and the work piece. The arc and molten metal are submerged in a blanket of granular fusible flux on the work piece. Filler metal is provided by the electrode (or from a supplemental source such as a welding rod or metal granules). The arc is covered by the flux.
Many sub arc applications are automatic welding applications where either the work piece is moved under the weld head or the weld head is moved over the stationary work piece. Such automatic systems include wire feeders and are well known in the art. Wire feeders used in sub arc welding may be either constant speed or variable speed. Constant speed wire feeders are typically used with CV power supplies, and variable speed wire feeders may be used with CC power supplies. Each type of wire feeder has advantages and disadvantages. Preferably, a welding power supply should be useable with a constant speed wire feeder, or useable with either type of wire feeder.
Early automatic sub arc welding applications provided a DC output and used power sources with drooping V-A characteristics and voltage following wire electrode feeders. Subsequently, constant voltage (CV) DC sources were introduced to the process and linked to constant speed wire electrode feeders. However, magnetic fields generated by the DC arc current and surrounding the arc and the field associated with the ground currents react with each other in an unpredictable manner, causing the arc to move as if the arc were being "blown" to one side. This is referred to as arc blow. This effect is most objectionable in deep grove welds where erratic movement of the arc disturbs proper formation and placement of the weld puddle. Arc blow becomes a more severe problem as the amperage increases, because magnetic fields correspondingly increase.
Arc blow is less of a problem when using an AC power supply (because there is not a DC arc current). However, a sinusoidal output does not always perform well in sub arc welding processes because the sinusoidal wave exhibits a slow zero crossover which may result in arc rectification.
Square wave welding power sources attempt to use the advantages of sinusoidal AC welding, but with a rapid zero crossing to avoid arc rectification. One known square wave welding power supply is described in U.S. Pat. No. 4,038,515 issued to Risberg. This power supply provides for a square wave AC welding output. The Risberg design provides a constant current (CC) output and thus cannot be used with a constant speed wire feeder. The output of this power supply is at a frequency equal to the input frequency.
Another prior art sub arc welding power supply is described in U.S. Pat. No. 4,322,602 which was issued to Grist, and was owned by the assignee of the present invention. Grist describes an AC constant potential (CV) power source which may be used for sub arc welding. The output of Grist is an AC/CV output having a frequency equal to the input frequency, and having a fast zero crossing. This power supply is used with a constant speed wire feeder.
A TIG (Tungsten inert gas) welding power supply is described in U.S. Pat. No. 5,340,963, which is also owned by the assignee of the present invention, and is hereby incorporated by reference. U.S. Pat. No. 5,340,963 shows an AC power source for welding which receives a three phase input and provides a single phase AC output, having relatively fast zero crossings, at a frequency 1.5 times the input frequency. This is a type of step-up cycloconverter. However, this prior art does not teach a CV mode of operation, nor a CC controller. This prior art can be operated in a DC mode, but only operates on half of the sinusoidal input (thus, the SCRs and secondary windings must be able to handle twice the current, relative to the current capacity needed if the entire input were used). This can be costly and add weight and size to the machine.
A "step-up cycloconverter", as used herein, is a cycloconverter having an output frequency greater than the input frequency. It receives an AC input at a given frequency and provides an AC output at a higher frequency. This conversion is obtained by phase control or without using switches that are forced off, such as force commutated SCRs, IGBTs or FETs. Thus, a rectifier followed by an inverter or buck/boost converter is not a cycloconverter. The applicants have learned that sub arc welding performed at a frequency greater than the input line frequency (50 or 60 Hz) will provide a better weld. Power sources that provide an output at greater that than 60 Hz are known and are generally inverters or other converters. However inverter based converters require the use of expensive switches that may be turned off, such as IGBT's. This is particularly true in applications such as sub arc welding where the current desired may exceed 1000 amps. Accordingly, inverter based power supplies for use in sub arc welding may be expensive and not practical.
Additionally, it is desirable to provide flexibility in a welding power supply so that it may be used for a variety of applications. For example, it is desirable to provide a welding power supply that provides an AC or DC output. Also, it is desirable to provide a welding power supply that provides either a CV or a constant current (CC) output, that may be used with a constant or variable speed wire feeder. Inverter based welding power supplies may be AC/DC and CC/CV, but as described above, they may be expensive, and not appropriate for sub arc applications.
Accordingly, it is desirable to provide a welding power supply that is suitable for sub arc welding that maybe operated either a CC, or a CV mode. Also, such a power supply preferably be operable to provide an output having a frequency greater than the input line frequency, when in the AC mode, but not require the use of IGBTs or other switches that may be turned off.